Introduce a context for rendering fixtures ERB.

I ran into this issue while trying to use binary data in my fixtures. The following guides motivated me to write this patch.

http://realityforge.org/code/rails/2006/04/06/loading-binary-data-into-rails-fixtures.html
http://www.tamingthemindmonkey.com/2011/08/18/rails-binary-data-in-fixtures
These define a new method right in the fixture file. I'm sure other people do this every once in a while too. With the current code, method definitions leak to other fixtures via the main object. This introduces subtle test inder-dependencies. Methods won't leak after this patch. (proven by new test case)

http://stackoverflow.com/questions/12644057/how-to-use-binary-data-in-rails-fixtures
This is a nice approach that's amenable to being packaged into a gem, except there is no clean place for adding this sort of functionality.

I look forward to your feedback.